During many normal biological interactions, various substances in a living organism must bind to one another through their receptor sites. At times, an individual's immune system can create an auto-immunity to one of these substances (often a binder or receptor protein) by production of blocking antibodies (so-called "auto blocking antibody"). The blocking antibodies are competitively attracted to the complementary binding sites on the receptor. Obviously, this reduces the quantity of binding sites available for binding with the complementary substance with which the receptor should normally interact (so-called "ligand"), also often a protein.
One well-known example of a potential auto blocking antibody system is the inter-relationship between intrinsic factor and cobalamine (vitamin B.sub.12). Intrinsic factor is a glycoprotein responsible for vitamin B.sub.12 absorption through the gastrointestinal tract. After ingestion, vitamin B.sub.12 binds to intrinsic factor through specific receptor sites and the complex is absorbed through the mediation of receptors for the complex in the gastrointestinal tract.
In the disease pernicious anemia, there is a gradual decline and eventual absence of the ability to make and secrete intrinsic factor. There is a very high association between pernicious anemia and the presence of auto-immune antibodies, particularly the intrinsic factor blocking antibody.
In situations of the type discussed above, it would be highly useful to be able to at least qualitatively determine the presence of the blocking antibodies, such as those produced for intrinsic factor (hereinafter "IF blocking site antibodies--at this time it is believed that intrinsic factor contains at least two types of binding sites, but only one of which is involved in forming the complex with vitamin B.sub.12).
Turning to the specific intrinsic factor-vitamin B.sub.12 situation, the inventors are aware of radioassay procedures used and suggested in the prior art for detecting the presence of IF blocking site antibodies. All of these prior art procedures are believed to involve solution techniques, that is, all reactive reagents are in solution or at least freely suspended within a liquid medium. Quite naturally, the approaches used are selected to take advantage of the ability of the IF blocking site antibodies to inhibit the binding of labeled (i.e., radioactive) vitamin B.sub.12 to intrinsic factor. In general, intrinsic factor (or gastric juice containing intrinsic factor) is admixed with the patient's biological fluid sample (ex serum) and then the labeled vitamin B.sub.12 (in excess) is added thereto. It is then necessary to separate bound labeled vitamin B.sub.12 from free labeled vitamin B.sub.12. Dialysis, gel filtration, charcoal absorption, and zirconyl phosphate gel absorption have been described as separatory techniques. Apart from these cumbersome and often time-consuming separatory procedures, the prior art solution-based radioassays present difficulties due to the presence of endogenous vitamin B.sub.12 binder in many patient samples. It is necessary to run an additional control to account for endogeneous vitamin B.sub.12 binder or to procedurally remove the endogenous binder from the sample.